1. Field of the Invention
The present invention relates to a photothermographic material and a method for producing silver halide to be employed therein.
2. Description of the Related Art
In recent years, it is strongly desired in the medical field to reduce the amount of waste processing liquids in view of environmental preservation and space saving. For this reason, there is desired a technology for a photothermographic material for medical diagnosis and for photographic applications, capable of efficient exposure with a laser image setter or a laser imager and of forming a sharp black image with a high resolution and a high sharpness. Such photosensitive thermal-development photographic material can eliminate use of processing chemicals in solutions and can provide users with a thermal development system which is simpler and does not contaminate the environment.
An image for medical use requires a particularly high image quality excellent in sharpness and graininess because a delicate image presentation is required. Also there is preferred an image of cold black tone in consideration of ease of diagnosis. Currently various hard copy systems utilizing pigments or dyes, such as an ink jet system and an electrophotographic system, are available as ordinary image forming systems, but no such system yet is satisfactory as an output system for the image for medical use.
On the other hand, a thermal image forming system utilizing an organic silver salt is disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075 and in “Thermally Processed Silver Systems”, B. Shely, Imaging Processes and Materials, Neblette 8th edition, edited by Sturge, V. Walworth and A. Shepp, p.2 (1996). More specifically, a photothermographic material has a photosensitive layer in which a catalytic active amount of a photocatalyst (for example silver halide), a reducing agent, a reducible silver salt (for example organic silver salt) and a toning agent for regulating the color of silver if necessary, are generally dispersed in a matrix binder. The photothermographic material is heated, after an exposure to an image, to a high temperature (for example 80° C. or higher) whereby a black silver image is formed by a redox reaction between the silver halide or reducible silver salt (acting as an oxidizing agent) and the reducing agent. The redox reaction is accelerated by a catalytic effect of a latent image in silver halide, generated by the exposure to light. Therefore, the black silver image is formed in an exposed area. Such system is described in various references including U.S. Pat. No. 2,910,377 and Japanese Patent Publication (JP-B) No. 43-4924, and a Fuji Medical Dry Imager FM-DPL has been commercialized as a medical image forming system utilizing a photothermographic material.
In the image forming system utilizing such organic silver salt, since the photosensitive silver halide or the reducing agent remains even after the development process because of the absence of a fixing step, an image preservability, particularly a gradual blacking of an unblackened area when exposed for example to an indoor light for a prolonged period (such phenomenon being called “print-out” in the present invention) has been a major issue. For alleviating such print-out, there has been proposed to use a halogen precursor compound or another development terminating agent, but such compound also has a drawback of inhibiting the image formation itself and cannot therefore be sufficiently effective. Also U.S. Pat. No. 6,143,488 and EP 0922995 disclose a method of utilizing AgI formed by converting the organic silver salt. However, such method cannot provide a sufficient sensitivity, and it is difficult to formulate a practically acceptable system. Also photosensitive materials utilizing AgI are disclosed in WO 97-48014 and 97-48015, U.S. Pat. No. 6,165,705, Japanese Patent Application Laid-Open (JP-A) No. 8-297345, Japanese Patent No. 2785129 etc., but none of these has attained a sufficient sensitivity and a sufficient fog level and has been practically usable.
In a photosensitive silver halide emulsion employed in a photothermographic material, it is well known a heavy metal doping can increase the sensitivity. For example JP-A No. 2001-42471 describes a doping of a transition metal of groups 6 to 10 of the periodic table in silver halide grains. It is also described that such doped metal is preferably distributed either at higher content at the surface or in the vicinity thereof of the grain than in the interior thereof, or uniformly throughout the grain, thereby contributing to an increase of the sensitivity.
JP-A No. 2001-042473 discloses a photosensitive silver halide emulsion having a high content phase of an iridium metal complex at a surface phase of silver halide grains. As an effect, there is obtained a high sensitivity.
Such heavy metal doping methods describe a heavy metal doping on the surface of silver halide grains, thereby obtaining a high sensitivity under an exposure with a high illumination intensity. However there has not been mentioned the issue of print-out which is specific to a photothermographic material, and it is estimated that the influence of the distribution of the doped heavy metal within the grain has not been recognized at all.
On the other hand, JP-A No. 2000-66325 discloses a method of doping an iridium-based dopant and a dopant based on a transition metal other than iridium in different areas of a silver halide grain respectively, and describes an effect of obtaining a higher sensitivity, particularly a sensitivity under an exposure of a high illumination intensity. There is described that the iridium-based dopant is doped in an area of 50 to 55% from the center of the silver halide grain, in an amount of 6×10−6 mol/mol.Ag, while the metal other than iridium is doped in an area of 65 to 70% from the center, in an amount of 4×10−5 mol/mol.Ag. It is also described that such method is applicable to a multi-color photosensitive material having two or more silver halide emulsion layers but a photothermographic material has not been mentioned. Consequently, it is considered that the issue of print-out, which is specific to the photothermographic material, has never been recognized.